1. Field of the Invention
The present invention relates microwave transmission in general, and more particularly to coupling elements to be used for coupling between respective microwave input and output conductors.
2. Description of the Related Art
There are already known various constructions of coupling elements to be interposed into microwave transmission lines, among them such that are widely used in the field of microwave technology and extensively described in the literature relating thereto and that combine the functions of a coupler with that of a bandpass filter by utilizing a pair of cooperating strip conductors in the respective coupling element. The typical strip conductor coupling element of this kind includes two parallel strip conductors that are galvanically uncoupled from each other and each of which has a length of xcex/4, wherein xcex is the effective nominal frequency of the microwaves that are to be transmitted through the coupling element. By resorting to the use of the Richard Transformation, which is described, for instance, in the book authored by Zinke and Brunswig and entitled xe2x80x9cLehrbuch der Hochfrequenztechnikxe2x80x9d, 1990, pages 206 to 211, such coupled xcex/4 conductors can be described by an equivalent circuit which includes a xcex/4 coaxial conductor with a wave propagation resistance of ZL connected between two capacitors. An ideal capacitive coupling element with minimum insertion loss can be provided if the wave propagation resistance ZL of the conductor in the equivalent circuit matches the wave propagation resistance in the two connected conductors to be coupled by the coupling element, in most instances 50xcexa9. The matching of the wave propagation resistance of the coupling element is accomplished by appropriately choosing the width and mutual distance of the parallel strip conductors of the coupling element. Using the assumed input and output conductor wave propagation resistance of 50xcexa9 and contemplating the use of the coupling element in a radar frequency range of approximately 24 Ghz, then, for use with a microwave substrate with a dielectric constant xcex5r=3.0 and a thickness of 250 xcexcm, there are required microstrip conductors with a width of about 90 xcexcm and a distance from one another of about 60 xcexcm. Yet, the production of strip conductors of these widths and mutual distances using the relatively inexpensive standard conductor plate technology is problematical at the very least.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a microwave coupling element that does not possess the drawbacks of the known coupling elements of this type.
Still another object of the present invention is to devise a microwave coupling element of the type here under consideration which has an insertion loss comparable with if not superior to that of the conventional coupling elements of this kind and, moreover, an improved frequency response, especially a highly selective bandpass characteristic.
It is yet another object of the present invention to design the above coupling element in such a manner as to be able to manufacture the same by resorting to the use or relatively inexpensive standard manufacturing techniques.
A concomitant object of the present invention is so to construct the microwave coupling element of the above type as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.
In keeping with the above objects and others which will become apparent hereafter, one feature of the present invention resides in a microwave coupling element for coupling an input conductor with an output conductor each exhibiting a predetermined wave propagation resistance. According to the invention, this coupling element includes a coupling portion interposed between the input and output conductors and including two parallel strip conductors and means for galvanically uncoupling the strip conductors from one another. The strip conductors of the coupling portion are spaced from each other by a predetermined distance and each has a predetermined width, at least one of the predetermined width and the predetermined distance being up to twice as large as that which would correspond to a minimum mismatch with the input and output connectors. Last but not least, the coupling element of the present invention includes means for compensating for the resulting mismatch, including at least one transformation connector exhibiting a wave propagation resistance smaller than the predetermined wave propagation resistance of the input and output conductors.
A particular advantage of the microwave coupling element of the present invention as described so far is that the strip conductors of the coupling portion can now be manufactured, owing to their relatively larger widths and/or spacing, by using standard conductor plate or integrated board manufacturing techniques rather then specialized, intricate and hence expensive procedures. As a result, there can be produced a relatively inexpensive microwave coupling element for use in the radar frequency range, above all for the mass production for instance in the motor vehicle manufacturing field. Yet, by proposing the use of at least one transformation conductor, the present invention avoids the mismatch and too high a wave propagation resistance, and with them the attendant increased insertion loss that would otherwise exist in reality and/or in the aforementioned equivalent circuit at these strip conductor widths and/or spacing.
According to an advantageous aspect of the present invention, there is further provided an additional transformation conductor similar to the one transformation conductor, the one and the additional transformation conductor being arranged between the input and output conductors, respectively, and the coupling portion. The use of such two transformation conductors has the advantage that, for the compensation of a certain higher coupling portion wave propagation resistance, a transformation conductor wave propagation resistance that is not all that small is sufficient for each of the two transformation conductors, so that a smaller width of the transformation conductor suffices. The advantageous width of a strip conductor is limited in the upward direction by transverse resonance effects and the like.
Advantageously, the two transformation conductors have a length of between one-fourth and one-eighth of the nominal wavelength of the coupling element. This length range constitutes an advantageous compromise between the overall length of the structural component, which should be as small as possible, and the electrical parameters that should be as close to ideal as possible.
When the coupling element of the present invention is to be used with input and output conductors with the predetermined wave propagation resistance amounting to 50xcexa9, it is advantageous when the wave propagation resistance of each of the transformation conductors is between 30 and 40xcexa9, preferably at about 35xcexa9.
According to another facet of the present invention, just one transformation conductor of the above kind is being used. In this instance, the transformation conductor advantageously has a length of about one-fourth of the nominal wavelength of the coupling element and the wave propagation resistance thereof is about a half of the predetermined wave propagation resistance. This implementation of the coupling element of the present invention has the advantage of a very short length of the overall coupling component.
The parallel strip conductors of the coupling portion advantageously have a length corresponding to one-fourth of the nominal wavelength of the coupling element. The predetermined width of each of the parallel strip conductors of the coupling portion advantageously is between 150 to 250 xcexcm, while the predetermined distance between the parallel strip conductors of the coupling portion advantageously lies between 100 and 200 xcexcm. Such dimensions can be easily produced in a relatively inexpensive manner by using standard integrated circuit board or conductor plate fabrication techniques.
Advantageously, the insertion loss at the nominal frequency of the coupling element is less than 1 dB. Moreover, the coupling element of the present invention advantageously exhibits a bandpass frequency response. What is especially desirable and achieved by the present invention is for the coupling element to exhibit a pronounced stop band attenuation of frequencies that are low with respect to the nominal frequency of the coupling element.
The microwave coupling element of the present invention further has the advantage that, as a result of the increased distance between the two strip conductors of the coupling portion, the resistance to dielectric breakdown is increased as well.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.